Molecular engineering strategies to control photo-induced birefringence and surface-relief gratings on layer-by-layer films from an azopolymer

The interactions governing adsorption of layer-by-layer (LBL) films from an azopolymer, PS-119(R), and poly(allylamine hydrochloride) (PAH) have been controlled by modifying the pH of the solutions used to form the films. The thickness per bilayer was varied by one order of magnitude, from 10 to 240 A, as the pH changed from 4 to 10. Thick layers were formed at higher pHs because in this case the PAH is only partially charged adopting a loopy conformation. This change in molecular conformation caused the kinetics of isomerization and mass transport to be much slower than at lower pHs. The writing time defined as the time to achieve 50% of maximum birefringence dropped from 110 to 18 min for films prepared from solutions with pH 10 and pH 4, respectively. This decrease is probably due to the higher free volume for isomerization in the films prepared at lower pHs, in which PAH molecules are less coiled than at higher pHs. For the same reason, the rate of inscription of surface-relief gratings with an interference pattern of p-polarized light was also much slower at higher pHs. (C) 2003 Elsevier B.V. All rights reserved.

Analysis of the kinetics of phase and amplitude gratings recorded in azopolymer films

In this work we use a stabilized holographic technique to study both refractive index and absorption gratings recorded in thin films made of Disperse Red 1 (DR1) embedded in an organic polymer matrix (PMMA) deposited on glass substrate. Gratings are recorded by linearly polarized illumination with the interference pattern of two crossing beams. One of the beams is phase modulated and the interference signals between the transmitted and diffracted waves are detected by a tuned lock-in amplifier. The technique allows measuring separately changes of the refractive index and the absorption coefficient during the course of the photoreaction process. The time evolution of the diffraction efficiencies during recording has shown bi-exponential kinetics for both gratings. © 2008 American Institute of Physics.

Running holograms in azopolymer films

We report investigations on running holograms recorded in an azopolymer film made of a poly(methyl methacrylate) matrix doped with Disperse Red 1. Two-wave mixing experiments were carried out in the symmetrical transmission geometry. A stabilization technique was employed for active control of the phase shift between the real-time hologram and the interference pattern. Depending on the imposed phase shift, a running hologram propagates in the material in the form of an isomerization wave created by a continuous erasing-rewriting process. Diffraction efficiencies and the hologram velocities were measured as functions of the holographic phase shift at the wavelengths 515 and 488 nm. The experimental results were compared to theoretical curves obtained from a simplified model of the isomerization kinetics. The selective contributions of the phase and the amplitude gratings to the whole hologram were also determined. © 2013 Springer-Verlag Berlin Heidelberg.

Instantaneous photoinduced patterning of an azopolymer colloidal nanosphere assembly

Colloidal azopolymer nanospheres assembled on a glass substrate were exposed to a single collimated laser beam. The combination of photo-fluidic elongation of the spherical colloids and light induced self-organization of the azopolymer film allows the quasiinstantaneous growth of a large amplitude surface relief grating. Pre-structuration of the sample with the nanosphere assembly supports faster creation of the spontaneous pattern. Confinement into the nanospheres provides exceptionally large modulation amplitude of the spontaneous relief. The method is amenable to any kind of photoactive azo-materials.

Instantaneous photoinduced patterning of an azopolymer colloidal nanosphere assembly

Colloidal azopolymer nanospheres assembled on a glass substrate were exposed to a single collimated laser beam. The combination of photo-fluidic elongation of the spherical colloids and light induced self-organization of the azopolymer film allows the quasiinstantaneous growth of a large amplitude surface relief grating. Pre-structuration of the sample with the nanosphere assembly supports faster creation of the spontaneous pattern. Confinement into the nanospheres provides exceptionally large modulation amplitude of the spontaneous relief. The method is amenable to any kind of photoactive azo-materials.

Synthesis of azopolymers with controlled structure and photoinduced birefringence in their LB films

The molecular architecture of azopolymers may be controlled via chemical synthesis and with selection of a suitable film-forming method, which is important for improving their properties for practical uses. Here we address the main challenge of combining the photoinduced birefringence features of azopolymers with the higher thermal and mechanical stabilities of poly(methyl methacrylate) (PMMA) using Atom Transfer Radical Polymerization (ATRP) to synthesize diblock- and triblock-copolymers of an azomonomer and the monomer methyl methacrylate. Langmuir-Blodgett (LB) films made with the copolymers mixed with cadmium stearate displayed essentially the same optically induced birefringence characteristics, in terms of maximum and residual birefringence and time for writing, as the mixed LB films with the homopolymer poly[4-(N-ethyl-N-(2-methacryloxyethyl))amino-2`-chloro-4`-nitroazobenzene] (HPDR13), also synthesized via ATRP. In fact, the controlled architecture of HPDR13 chains led to Langmuir films that could be more closely packed and reach higher collapse pressures than the corresponding films obtained with HPDR13-conv synthesized via conventional radicalar polymerization. This allowed LB films to be fabricated from neat HPDR13, which was not possible with HPDR13-conv. The enhanced organization in the LB films produced with controlled azopolymer chains, however, led to a smaller free volume available for isomerization of the azochromophores, thus yielding a lower photoinduced birefringence than in the HPDR13-conv films. The combination of ATRP synthesis and LB technology is then promising to obtain optical storage in films with improved thermal and mechanical processabilities, though a further degree of control must be sought to exploit film organization while maintaining the necessary free volume in the films. (C) 2008 Elsevier Ltd. All rights reserved.

Photoinduced phenomena in layer-by-layer films of poly(allylamine hydrochloride) and Brilliant Yellow azodye

The nanoscale interactions between adjacent layers of layer-by-layer (LBL) films from poly(allylamine hydrochloride) (PAH) and azodye Brilliant Yellow (BY) have been investigated, with the films employed for optical storage and the formation of surface-relief gratings. Using Fourier transform infrared spectroscopy, we identified interactions involving SO3- groups from BY and NH3+ groups from PAH. These electrostatic interactions were responsible for the slow kinetics of writing in the optical storage experiments, due to a tendency to hinder photoisomerization and the subsequent reorientation of the azochromophores. The photoinduced birefringence did not saturate after one hour of exposure to the writing laser, whereas in azopolymer films, saturation is normally reached within a few minutes. on the other hand, the presence of such interactions prevented thermal relaxation of the chromophores after the writing laser was switched off, leading to a very stable written pattern. Moreover, the nanoscale interactions promoted mass transport for photoinscription of surface-relief gratings on PAH/BY LBL films, with the azochromophores being able to drag the inert PAH chains when undergoing the trans-cis-trans photoisomerization cycles. A low level of chromophore degradation was involved in the SRG photoinscription, which was confirmed with micro-Raman and fluorescence spectroscopies.

Estudos de birrefringência fotoinduzida em filmes LB mistos do azopolímero poli (metacrilato de lauroila-co-metacrilato de DR1) com estearato de cádmio

Pós-graduação em Ciência e Tecnologia de Materiais - FC

Inverse relaxation effect of azo-dye molecules: The role of the film anisotropy

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Langmuir and Langmuir-Blodgett films of a quinoline-fluorene based copolymer

The fabrication of Langmuir-Blodgett (LB) films of synthetic polymers allows the control of molecular architecture in order to optimize physical properties. In this paper, the surface chemistry of a quinoline-fluorene based copolymer spread on the air-water interface is investigated. Surface pressure-area isotherms as well as Polarization-Modulation Infrared Reflection-Absorption Spectroscopy (PM-IRRAS) were employed to characterize the films, which could be transferred to solid supports by the LB technique. Atomic force microscopy as well as UV-Vis and fluorescence spectroscopies have shown a regular deposition of the polymers, and the luminescence properties could be controlled with the number of layers deposited on the solid support. As a result, the photoluminescence of the LB films was considerably higher than that observed for the spin coated film, and the maximum emission peak was shifted to higher energies, which is attributed to the molecular-level interactions within the layer-ordered structure of the LB film. The luminescence response would possibly be tuned to approach the highest level, which allows the films to be employed in future applications in efficient optical devices such as organic light-emitting diodes (OLEDs). (C) 2011 Elsevier B.V. All rights reserved.

Design of Transparent Conducting Electrodes and Surface Relief Gratings for Plasmonic Polymer Solar Cells

As the concept of renewable energy becomes increasingly important in the modern society, a considerable amount of research has been conducted in the field of organic photovoltaics in recent years. Although organic solar cells generally have had lower efficiencies compared to silicon solar cells, they have the potential to be mass produced via solution processing. A common polymer solar cell architecture relies on the usage of P3HT (electron donor) and PCBM (electron acceptor) bulk heterojunction. One of the main issues with this configuration is that in order to compensate for the high exciton recombination rate, the photoactive layer is often made very thin (on the order of 100 $%). This results in low solar cell photocurrents due to low absorption. This thesis investigates a novel method of light trapping by coupling surface plasmons at the electrode interface via surface relief gratings, leading to EM field enhancements and increased photo absorption. Experimental work was first conducted on developing and optimizing a transparent electrode of the form &'()/+,/&'() to replace the traditional ITO electrode since the azopolymer gratings cannot withstand the high temperature processing of ITO films. It was determined that given the right thickness profiles and deposition conditions, the MAM stack can achieve transmittance and conductivity similar to ITO films. Experimental work was also conducted on the fabrication and characterization of surface relief gratings, as well as verification of the surface plasmon generation. Surface relief gratings were fabricated easily and accurately via laser interference lithography on photosensitive azopolymer films. Laser diffraction studies confirmed the grating pitch, which is dependent on the incident angle and wavelength of the writing beam. AFM experiments were conducted to determine the surface morphology of the gratings, before and after metallic film deposition. It was concluded that metallic film deposition does not significantly alter the grating morphologies.

Design of Transparent Conducting Electrodes and Surface Relief Gratings for Plasmonic Polymer Solar Cells

As the concept of renewable energy becomes increasingly important in the modern society, a considerable amount of research has been conducted in the field of organic photovoltaics in recent years. Although organic solar cells generally have had lower efficiencies compared to silicon solar cells, they have the potential to be mass produced via solution processing. A common polymer solar cell architecture relies on the usage of P3HT (electron donor) and PCBM (electron acceptor) bulk heterojunction. One of the main issues with this configuration is that in order to compensate for the high exciton recombination rate, the photoactive layer is often made very thin (on the order of 100 $%). This results in low solar cell photocurrents due to low absorption. This thesis investigates a novel method of light trapping by coupling surface plasmons at the electrode interface via surface relief gratings, leading to EM field enhancements and increased photo absorption. Experimental work was first conducted on developing and optimizing a transparent electrode of the form &'()/+,/&'() to replace the traditional ITO electrode since the azopolymer gratings cannot withstand the high temperature processing of ITO films. It was determined that given the right thickness profiles and deposition conditions, the MAM stack can achieve transmittance and conductivity similar to ITO films. Experimental work was also conducted on the fabrication and characterization of surface relief gratings, as well as verification of the surface plasmon generation. Surface relief gratings were fabricated easily and accurately via laser interference lithography on photosensitive azopolymer films. Laser diffraction studies confirmed the grating pitch, which is dependent on the incident angle and wavelength of the writing beam. AFM experiments were conducted to determine the surface morphology of the gratings, before and after metallic film deposition. It was concluded that metallic film deposition does not significantly alter the grating morphologies.